Allows scons vcxproj to work on all systems

This changes the way toolchains is looped over, always looping over
only gcc, clang, and msvc, but only aliasing if the toolchain is in
toolchains.

Changes along the way:
* Ensures the visual studio file is always described
* Makes the toolchain loop more straightforward
* Removes an old variable
* Prevents checking nonexistant enviro vars
* Prevents pkg-config protobuf env vars if not clang/gcc
* Removes tabs
* Doesn't use any pkg-config enviro vars unless clang/gcc
* Makes all directories windows-style